Quorum sensing is one mechanism by which many pathogenic bacteria coordinate gene expression. At high cell densities, signaling by the las and rhl quorum sensing systems of Pseudomonas aeruginosa induces the expression of approximately 300 genes, including numerous virulence genes. With addition of exogenous quorum sensing signals, some genes, such as hcnABC (encoding hydrogen cyanide synthase) are induced early in growth while others, such as lasB (encoding elastase) are expressed late in growth. I propose that the affinity of quorum-controlled gene promoters for the transcriptional regulators encoded by the quorum systems determines the timing of their induction. The contribution of individual regulator binding sites within the lasB and hcnA promoters to induction timing will be determined by deletion, mutation, or rearrangement of these sites in the presence of varying levels of the transcriptional regulator encoded by the las system, LasR. LasR will be purified and used in gel shift assays to determine the affinity of the lasB and hcnA promoters for LasR, in DNAse protection assays to assess potential cooperative binding of LasR by multiple sites in the lasB and hcnA promoters, and in in vitro transcription assays. Affinity of these promoters for LasR will then be correlated to induction timing.